1. Field of the Invention (Technical Field)
This invention focuses on the integration of an advanced heat spreader utilizing a closed loop heat pipe containing various “working fluids” like acetone, water and liquid nitrogen to provide greatly improved thermal management for solid-state laser including thin disk lasers, planar waveguide, and fiber lasers plus both edge emitting stacked bar laser diodes and vertical-cavity surface-emitting laser (VCSEL) diodes. The initial emphasis was projected most suitable for the lasing thin disk which are nominally 200 microns (10−6 meters), 1-2 cm in diameter and crystalline/ceramic material like Yb:YAG, tungstates, sesquioxides and others, the immediate practical use of this integration should have very good benefits. The emphasis of this enhanced thermal management of all types of solid-state laser systems is an approach to achieve much more uniform and efficient conduction for the lasing solid-state systems at high powers to significantly improve its beam quality (BQ) thus enabling much more widespread use. Besides the solid-state laser system itself, there are several other uses such as high power mirrors and other HEL infrastructural components including pumping laser diodes and transmissive “conditioning” optics of the laser diode pump radiation. The AHS system coupled with a mechanical-control flow of the oscillating heat pipe flow can be applied to lasing laser diodes, semiconductor laser, quantum cascade, and heterostructures lasing system plus nonlinear optical components. Also, this AHS integrated with many all types of solid-state lasers can be operated at room temperature and sub-zero temperature down to below 80° K.
2. Description of Related Art
Note that the following discussion may refer to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
High power solid-state lasers (SSL) have become the emphasis for industrial and commercial applications due to their efficiency, compactness and small supporting infrastructure relative to high power electrical or chemical gas laser system. Scaling SSL to high powers (>kW's and approaching 100 kW's) has required specific attention to thermal management. Over the last 15 years, thin disk laser has been developed and matured in Germany into reliable laser systems operating at kW's of average powers and used extensively for welding, cutting and material processing. In spite of this success, there are several issues needed to be improved to advance TDL technology to next level, including (1) thermal management; (2) power limitations by amplified spontaneous emission (ASE); (3) thin-disk gain material fabrication for reliable operation and (4) improved homogenization of the pumping laser diode radiation to acquire nearly perfect “flattop” intensity profiles. This invention initially focuses in the near term on the thermal management aspect of the lasing thin disk which is nominally 200 microns (10−6 meters), 1-2 cm in diameter and crystalline/ceramic material like Yb:YAG, tungstates, sesquioxides and others. In addition, all of the advances in TDL using this novel and very unique heat spreader can be applied directly to many other type of high power solid-state laser systems.
The thin disk laser (TDL) pioneered by Giesen has demonstrated high powers (>4-5 kW) and “wall plug” efficiencies better than 20% as discussed by A. Giesen, et al., “Scalable concept for diode-pumped high-power solid-state lasers”, Appl. Phys., Vol. B 58, 363 (1994) and J. Speiser, “Fifteen years of work on thin-disk lasers: results and scaling laws”, IEEE J. Sel. Top. Quant. Electr., Vol. 13 (3), p. 598 (2007). FIG. 1 illustrates the concept consisting of a lasing thin disk (TD), a hemispherical resonator having back side of thin disk serving as a flat mirror, a cooled heat sink for the TD and the laser diode pump radiation coming from a parabolic multi-pass reflector assembly not shown as disclosed in U.S. Pat. Nos. 6,438,152; 6,577,666; 6,963,592; 7,003,011; and 7,200,160. Although only a part of the pump beam is absorbed by the thin gain element, the pump efficiency can be optimized by re-imaging the pump radiation several times with an optical system of a parabolic mirror and redirecting mirrors. Up to 32 passes of the pump beam can be used with more than 90-98% of the pump power absorbed in the disk. The output laser power at kW levels is multi-transverse mode with M2 values from 5-20 and single, lowest order mode cw operation occurs only for a few 100's of watts.
Even with these impressive results, it is important to achieve much higher single mode laser operation with good beam quality (BQ) from thin disk lasers. Improved high power BQ operation of TDL would enable many more applications. The diminished BQ performance at high powers, however, is the most significant shortcoming of this promising laser technology and is caused by the thermal behavior of the lasing thin disk. As the TD is heated by the optical pumping radiation, it experiences non-uniform temperature profiles, which creates a dynamic lens plus significant thermally-induced stress. These stresses produce both time and spatial varying birefringence in the disk that causes laser polarization changes and non-spatial phase changes across the thin disk. This polarization change and the phase changes, respectively, reduce the laser energy extraction from the thin disk and further degrade the TDL beam quality. All of these thermal problems are attributed to insufficient thermal management of the thin disk. To date, nearly all high power TDL use jet impingement cooling which produces a spatially non-uniform cooling of the thin disk. In this patent application, the use of a special variation of heat pipes produces significantly improved isothermal cooling of the thin disk that will lead to greatly improved beam quality plus good energy extraction from the thin disk laser. This invention should greatly expand the application of the TDL.
The main difference between TDL and conventional rod or slab lasers is the geometry of gain medium. For the TDL, the thickness of the laser crystal (or ceramic) is quite small, 100-200 μm and the diameter is typically 1-2 cm. The large surface-to-volume ratio of the thin disk like fiber lasers makes possible the efficient removal of heat from the TD. As heat is removed via diffusion through the back side of the TD, the temperature distribution in the radial direction could be made quite uniform provided the central area of the disk is pumped by a near flat-top intensity profile and the diffusive cooling is very efficient. Today's operating TDL, however, do not have these ideal pumping and cooling conditions. The present invention deals with the thermal management of the thin disk in an attempt to achieve much more uniform and efficient conduction cooling for the TD's. The most commonly employed approaches for the thin disk lasers use jet impingement cooling of two configurations shown in FIG. 2. These types are the “cold plate” (or “cold finger”) thin disk and the “capped” thin disk. In the former, the thin disk is bonded to a “cold plate” (usually 1 mm thick CuW for Yb:YAG lasing thin disk) which is cooled on the back side via water jet impingement. [D. J. Womac, “Correlating Equations for Impingement Cooling of Small Heat Sources with Single Circular Liquid Jets”, Transactions of ASME, Vol. 115, p. 106 (1993)] In the “capped” thin disk, an approximate 1 mm thick, undoped crystal or polycrystalline ceramic (like YAG) is bonded to a thin disk like Yb:YAG. The Yb:YAG is then cooled directly to maximize the diffusion of heat through the 200 micron thick lasing material (Yb:YAG is this case) as disclosed in U.S. Pat. Nos. 6,600,763 and 7,200,160. The 1 mm thickness material provides mechanical strength for the nominal 200 micron laser gain material. The most widely used gain medium for TDL is Yb:YAG as discussed by C. Stewen, et al, “A 1-kW cw thin disc laser”, IEEE J. Sel. Top. Quant. Electr., Vol. 6 (4), p. 650 (2000); K. Contag et al., “Theoretical modeling and experimental investigations of the diode-pumped thin disk Yb:YAG laser”, IEEE J. Quant. Electr., Vol. 29 (8), p. 697 (1999); D. Kouznetsov, et al., “Surface loss limit of the power scaling of a thin-disk laser”, J. of Opt. Soc. Amer., Vol. B-23 (6), p. 1074 (2006). Lately many other system has been demonstrated like Yb:KGW as discussed by G. Paunescu, et al., “100-fs diode-pumped Yb:KGW mode-locked laser”, App. Phys. B, Vol. 79, p. 555 (2004); J. E. Hellstrom, et al., “Efficient Yb:KGW lasers end-pumped by high-power diode bars”, Appl. Phys. B, Vol. 83, p. 235 (2006), Yb:KYW as discussed by K. Seger, et al., “Tunable Yb:KYW laser using a transversely chirped volume Bragg grating”, Optics Express, Vol. 17 (4), p. 2341 (2009)] and Yb:Lu2O3 as discussed by R. Peters, et al., “Broadly tunable high-power Yb:Lu2O3 thin disk laser with 80% slope efficiency”, Opt. Express, Vol. 15 (11), p. 7075 (2007).
Unfortunately, as related above, non-optimum conduction cooling creates a non-uniform uniform temperature across the thin disk surface and in the TD itself as shown in FIG. 3 for the two specific geometric configurations shown in FIG. 2. FIG. 4 illustrates the analysis details and are described later. Such non-isothermal temperature profile conditions cause a thermal lensing effect plus thermal-stress induced birefringence which results in the laser power decreasing due to depolarization loss. A thin disk of a high power TDL has very high power loading in the disk (up to 100's of kW/cm3 absorbed pump power density). To date, approximately 100 W of output power in a diffraction-limited continuous-wave (CW) beam has been obtained with a single disk. Multimode TDL with 6.5 kW output power from a single disk has been demonstrated. [A. Lobad, et al., “Characterization of a Multikilowatt Yb:YAG Ceramic Thin-Disk Laser”, J. of Directed Energy, March, 2011] The scaling laws show that the power limit for CW operation can be theoretically projected to be beyond 40 kW for one single disk. [J. Speiser, “Fifteen years of work on thin-disk lasers: results and scaling laws”, IEEE J. Sel. Top. Quant. Electr., Vol. 13 (3), p. 598 (2007)] These non-uniform temperature profiles in a lasing crystal disk result in thermal distortion of the output beam and degradation of laser operation due to thermal stress induced birefringence and deformation in the thin disk. FIGS. 3 and 5 show the non-uniform temperature profile in the radial direction and the temperatures increases in the supporting structure for the TD. In FIG. 5, note the significant difference between the “cold-finger” and “capped” configurations of the Yb:YAG thin disk across the radial direction. These temperature variations for the entire thin disk holding assembly also cause significant deformation for the entire structure. FIG. 6 illustrates this effect for both TD configurations. FIG. 7 shows the axial expansion in a 1 cm pumped cross section versus the radius. Any laser beam propagating through this thermally induced lens will create significant detrimental effects on the TDL's ultimate BQ and resulting performance.